Case study by Des Davies AAE MIMI, Top Gear Motor Services Vehicle: Vauxhall Mokka 1.4 LUJ/B14NET Turbo 2018
Fault Code: P0300
Customer complaint: Loss of power, feels like lack of fuel and engine running poorly
I interrogated the customer to get as much detail and information on the symptoms, leading up to and when the faults first developed. I then visually inspected the engine bay for any anomalies or obvious faults.
I confirmed the fault – the engine was misfiring badly, and the check engine light was displayed on the instrument panel. There was no need for a road test as it was running poorly on idle speed.
I connected my scanner to the OBD port to check for fault codes to get some clues that may help diagnose the fault. It flagged up a ‘P0300 Engine Misfire Detected’, so not much help from this generic code.
My next plan of attack was to work smarter not harder by going global and using the OBD11 on my scanner and checking Mode 6 as this has helped me in the past. This mode can be useful as Vauxhall have misfire counters in Test Mode 1.
Mode 6 global information, see Figure 1, recognised cylinder no.4 as the misfiring offender so it was time to break out my PicoScope and investigate. I needed to confirm cylinder no.4 was indeed misfiring as this data can be incorrect. Don’t always trust what you see, I have been burnt before.
Checking the essentials…
For a good combustion burn we need a good spark, at the right time, with the correct amount of air and fuel at the correct time, and good compression. If we have all these requirements then the engine should start and run efficiently.
Planning my next diagnostic approach, I could do a quick relative compression test which is non-intrusive and a good procedure to confirm that the engine mechanicals are in good working order, but bear in mind that it only compares the cylinder work rate against each other and is not an indication of good cylinder compression.
I disabled the fuel to prevent the vehicle from starting and needed to locate the fuel relay or fuse by extracting this information from a technical data source. Figure 2 shows the good relative compression waveform.
The relative compression test confirmed that all cylinders were mechanically distributing an equal amount of engine compression, so I decided to test the next easiest system, because of its ease of access, the ignition coil pack assembly.
Working smarter not harder, I now needed a wiring diagram on this system to help me test the ignition system and to confirm the correct wiring for this vehicle, see Figure 6.
These ignition systems use an ignition coil pack, also called cassettes, where the four ignition coils are housed as one coil unit. Each coil is individually controlled and switched with a square wave signal wire by the ECM.
Unfortunately, the diagram information did not give me the wiring locations from the ECM pins to this coil pack. I needed to locate the terminal for no4, so I had to test all the ECM coil driver wires to confirm that they were being switched by the ECM coil driver and were all operating correctly.
We now needed to take a closer look at cylinder no.4’s coil secondary output extension lead and the spark plug to check for any anomalies. In Figure 10, we can see evidence of the results of the spark plug firing voltage for no.4 with carbon tracking on the spark plug insulator arcing across the spark plug terminal to the cylinder head, causing the plug not to fire inside the engine cylinder and causing our misfire.
A new coil pack was required as well as new spark plugs. We then needed to retest the ignition system waveforms to confirm the faulty coil pack and spark plugs had fixed the misfire. Figure 13 shows the waveform of the secondary ignition system after fitting the new ignition components. Figure 14 shows the ignition coil pack current was good.